The following description of background art may include insights, discoveries, understandings or disclosures, or associations together with disclosures not known to the relevant art prior to the present invention but provided by the invention. Some of such contributions of the invention may be specifically pointed out below, whereas other such contributions of the invention will be apparent from their context.
Printed papers particularly those having a fairly dark single color on a large area may appear spotted or cloudy. Such a phenomenon is called mottling and it may be caused by various reasons such as bad formation, small scale variation of coating, uneven absorption of ink or the like. Irrespective of the physical reason, small scale variation of gloss of paper before printing is known to result in mottling in the printed paper.
Gloss is visual appearance of material such as paper, carton and board, and it has a close relation with surface texture. Surface gloss is conventionally measured as a specular reflection of visible light from a relatively large area in the order of at least tens of square millimeters. In addition to reflection from the surface, scattering from inside the object may also have some effect to the value of the measured surface gloss.
Lately, a measurement of micro gloss which is also measured as a specular reflection has become important particularly in paper industry. The micro gloss is a two-dimensional representation of a reflection of light from a small measured area, the area being from about a few square micrometers to tens of square millimeters. An idea of measuring micro gloss has been considered possible such that a camera with a detector of a matrix of pixels captures images of a moving surface which is illuminated with stroboscopic flashes. Because small area may be captured in the image, resolution in the order of 100 μm could be reached.
However, the measurement faces problems. For example, the captured image of the web is slightly inaccurate since the web is moving during exposure with the stroboscopic flashes. Moreover, the measurement configuration with a camera having a detector matrix, a stroboscopic flash light, and optical components is complicated. Hence, there is a need for better measurement of small scale gloss.